The disclosure relates generally to engines, and more specifically to a method of operating an engine to increase the range of speeds and loads under which the engine can operate.
Compression-ignition engines, such as diesel engines, operate by directly injecting a fuel (e.g., diesel fuel) into compressed air in one or more piston-cylinder assemblies, such that the heat of the compressed air ignites the fuel-air mixture. The direct fuel injection atomizes the fuel into droplets, which evaporate and mixes with the compressed air. Compression-ignition engines may also be configured to operate under a premixed combustion operation or partially premixed combustion operation. Typically, compression-ignition engines operate at a relatively higher compression ratio than spark ignition engines. The compression ratio directly affects the engine performance, efficiency, exhaust pollutants, and other engine characteristics. In addition, the fuel-air ratio affects engine performance, efficiency, exhaust pollutants, and other engine characteristics. Exhaust emissions generally include pollutants such as carbon oxides (e.g., carbon monoxide), nitrogen oxides (NOx), sulfur oxides (SOx), and particulate matter (PM). In a compression-ignition engine, the amount and relative proportion of these pollutants varies according to the fuel-air mixture, compression ratio, injection timing, environmental conditions (e.g., atmospheric pressure, temperature, etc.), and so forth.
A dual-fuel engine is an alternative type of internal combustion engine designed to run on more than one fuel, for example, natural gas and diesel, each stored in separate vessels. Dual fuel engines typically operate at much lower combustion temperatures because of improved combustion control and reduce tendency to knock. Furthermore, dual fuel engines result in less energy loss from the engine through heat transfer than found in a typical engine. More particularly, such engines are capable of burning varying proportions of the resulting blend of fuels in the combustion chamber and the fuel injection or spark timing may be adjusted according to the blend of fuels in the combustion chamber to result in premixed combustion. In addition, customization of the fuel blend with respect to fuel chemistry provides for optimal combustion and results in less unburned fuel energy lost in the exhaust and fewer pollutant emissions during the combustion process. For dual fuel operation where one of the fuels is premixed with air, a reduction in nitrogen oxide (NOx) and particulate matter (PM) emissions is enabled by combusting a relatively larger fraction of the premixed fuel.
Premixed combustion strategies, such as those previously described, are of interest in internal combustion engines due to the significant reduction in NOx and PM emissions while often times enabling improvements in efficiency. However, one challenge that plagues these types of engines is operation at high loads. For premixed combustion, the fuel-air mixture needs to be supplied at approximately the same stoichiometry, regardless of the load. That means that for constant intake temperature, as the load increases, the peak cylinder pressures increase. However, when the engine is operated at high load, uncontrolled auto-ignition of the premixed fuel can become a problem and peak cylinder pressures become very high, limiting premixed combustion strategies to lower loads only.
Accordingly, there is a need for an improved system and method for engines operating under a premixed combustion configuration that permits engine operation at increased speeds and under higher load conditions without causing auto-ignition and reaching peak firing pressure limits.